In recent years, reductions in the weight of automobile bodies are being sought with the aim of improving the fuel consumption of automobiles to contribute to protection of the global environment. In addition thereto, since it is necessary to ensure the safety of passengers, there is also a growing need for high-strength steel sheets. Recently, steel sheets having a tensile strength of 1470 MPa or more are also desired. However, a steel sheet to serve as a member for an automobile is required to not only have a high strength, but also to have favorable press-formability.
It is necessary to improve the ductility of a steel sheet in order to improve the press-formability thereof. However, tensile strength and ductility are elements that are contrary to each other, and it is usually difficult to improve both tensile strength and ductility at the same time. In addition, members for automobiles are required to have corrosion resistance and also continuous spot weldability with respect to spot welding. Therefore, it is necessary to achieve high strength and favorable press-formability by using a galvannealed steel sheet.
A steel sheet that utilizes a TRIP effect of retained austenite is known as a steel sheet having favorable uniform elongation while also having a high strength. For example, Patent Document 1 discloses a high-strength galvannealed steel sheet having good workability and a method for producing the high-strength galvannealed steel sheet.
Patent Document 2 discloses a high-strength galvannealed steel sheet having a tensile strength of 590 MPa or more that includes an alloyed hot-dip galvanized layer that is excellent with respect to wettability of plating and adhesion of a plated layer, and a method for producing the high-strength galvannealed steel sheet.
Patent Document 3 discloses a high-strength galvannealed steel sheet having excellent workability and corrosion resistance, and a method for producing the high-strength galvannealed steel sheet. In addition, Patent Document 4 discloses a steel sheet for molding members that has ultra-high strength which is achieved by heat treatment and also having excellent ductility, as well as a molding member and a method for producing the steel sheet.